Rotational moulding of thermoplastics material is a convenient and cost effective method for the manufacture of a wide variety of articles, ranging in size from domestic containers and toys to large liquid storage tanks.
Rotational moulding processes are characterised by low capital costs for molds and other apparatus when compared with injection moulding processes for example. They are further characterised by relatively short setup times, the capacity for economic low volume manufacturing and immense flexibility in product size range.
Typical thermoplastics materials employed in rotational moulding processes are the polyolefins, including polyethylene and polypropylene, polyvinyl-chloride, polycarbonates, nylon, and acrylobutadiene-styrenes (ABS). In this respect virtually any thermoplastic resins are suitable for use in rotational moulding, including copolymeric materials and mixtures of compatible resins.
Products produced by rotational moulding are generally characterised by good strength and structural integrity, abrasion resistance, weather resistance including UV stability, wide colour range, selectable surface finish from high gloss to matt and textured, chemical resistance and environmental stress crack resistance (ESCR).
Although generally satisfactory for most purposes, in certain applications rotationally moulded products lack stiffness, impact resistance (particularly at low temperatures), and load bearing capacity (particularly at elevated temperatures). These deficiencies generally arise from the relatively low wall thickness of products produced by rotational moulding which are inherently due to thermal conductivity and cost constraints.
Growing environmental concerns over the disposal of plastics waste has focussed the need for recycling of plastics materials reclaimed subsequent to use by domestic or industrial consumers. One of the major difficulties in recycling such reclaimed plastics is contamination. Contamination can arise from inadequate sorting of polymeric species which is difficult to achieve by unskilled manual labour and virtually impossible to automate.
Mixtures of plastics species are difficult to process due to differing melt temperatures and viscosities as well as inherent chemical incompatibility. Other sources of contamination, apart from dirt and dust, are paper and plastics film labels, non-plastics container seals and incompletely discharged contents of such containers.
A method for manufacturing products using reclaimed plastics, typically polyolefins, employs an injection extrusion process whereby granulated plastics waste is subjected to working under high shear conditions in an extruder. The compounded plastics issuing from the extruder is pumped directly, under screw pressure, into a shaped hollow die and allowed to cool, in a manner similar to injection moulding.
The injection extrusion of reclaimed plastics is generally used to produce articles such as plank members for outdoor seating, tree and shrub surrounds, garden edging, man-hole covers and pits for underground utilities or shipping pallets. The articles are characterised by high mass, bulky shape, limited colour range, poor chemical resistance, poor ESCR and UV resistance along with inferior mechanical properties. The surface of such articles is uneven or porous and thus difficult to keep clean. Textured finishes, as required for slip resistance on man-hole covers, are difficult to reproduce.
These characteristics of reclaimed plastics products result from contamination and chemical incompatibility problems and the use of a dark pigment, usually black brown or grey, to mask unsightly flow marks from inadequately blended plastics of differing colours and viscosities. A large amount of expensive pigment must be added to achieve even a reasonable colour finish.
The applicant's Australian Patent Application No. 40440/95 describes a method for the manufacture of moulded plastics articles which combines certain advantages of conventional rotational moulding with certain advantages of injected cores to selectively overcome disadvantages of each process.
The applicant's earlier method comprises a two cycle method of first rotationally moulding a thin walled hollow body and secondly introducing into the core of said hollow body a flowable mass of plastics material, preferably reclaimed plastics material, to occupy substantially the entire volume of the core of said hollow body. This method is particularly suited to producing relatively large thin walled objects such as buoyancy modules or liquid storage tanks.
Whilst suitable for thin walled objects, the applicant's earlier method consumes a large amount of reclaimed plastics in the production of articles that have a core volume commensurate with the overall size of the article, thus resulting in a high mass product.
Consequently there is a need for an improved method which incorporates the benefits of the double skin structure of the prior art method but utilises less material to produce articles that are lighter in weight whilst maintaining their structural integrity. Furthermore it would be advantageous if the method required only a single cycle rotational moulding process, rather than the prior art two cycle method of forming the outer skin followed by introducing the flowable reclaimed plastics material to form the core.
The improved method is suited to the production of bulky articles, such as pallets, lids for industrial waste bins and tanks, covers for utility access ways, pits (eg. for underground cable or conduit systems), or structural members such as wall, roof, or floor panels.